Sodium Iron(II)-Hexacyanoferrate(II) Battery Electrode

ABSTRACT

A method is provided for synthesizing sodium iron(II)-hexacyanoferrate(II). A Fe(CN) 6  material is mixed with the first solution and either an antioxidant or a reducing agent. The Fe(CN) 6  material may be either ferrocyanide ([Fe(CN) 6 ] 4− ) or ferricyanide ([Fe(CN) 6 ] 3− ). As a result, sodium iron(II)-hexacyanoferrate(II) (Na 1+X Fe[Fe(CN) 6 ] Z .MH 2 O is formed, where X is less than or equal to 1, and where M is in a range between 0 and 7. In one aspect, the first solution including includes A ions, such as alkali metal ions, alkaline earth metal ions, or combinations thereof, resulting in the formation of Na 1+X A Y Fe[Fe(CN) 6 ] Z .MH 2 O, where Y is less than or equal to 1. Also provided are a Na 1+X Fe[Fe(CN)6] Z .MH 2 O battery and Na 1+X Fe[Fe(CN) 6 ] Z .MH 2 O battery electrode.

RELATED APPLICATIONS

This application is a Divisional of an application entitled, SODIUMIRON(II)-HEXACYANOFERRATE(II) BATTERY ELECTRODE AND SYNTHESIS METHOD,invented by Yuhao Lu et al., Ser. No. 14/067,038, filed Oct. 30, 2013,attorney docket No. SLA3315;

which is a Continuation-in-Part of an application entitled, TRANSITIONMETAL HEXACYANOMETALLATE-CONDUCTIVE POLYMER COMPOSITE, invented by SeanVail et al, Ser. No. 14/059,599, filed Oct. 22, 2013, attorney docketNo. SLA3336;

which is a Continuation-in-Part of an application entitled, METAL-DOPEDTRANSITION METAL HEXACYANOFERRATE (TMHCF) BATTERY ELECTRODE, invented byYuhao Lu et al., Ser. No. 13/907,892, filed Jun. 1, 2013, attorneydocket No. SLA3287;

which is a Continuation-in-Part of an application entitled,HEXACYANOFERRATE BATTERY ELECTRODE MODIFIED WITH FERROCYANIDES ORFERRICYANIDES, invented by Yuhao Lu et al., Ser. No. 13/897,492, filedMay 20, 2013, attorney docket No. SLA3286;

which is a Continuation-in-Part of an application entitled, PROTECTEDTRANSITION METAL HEXACYANOFERRATE BATTERY ELECTRODE, invented by YuhaoLu et al., Ser. No. 13/872,673, filed Apr. 29, 2013, attorney docket No.SLA3285;

which is a Continuation-in-Part of an application entitled, TRANSITIONMETAL HEXACYANOFERRATE BATTERY CATHODE WITH SINGLE PLATEAUCHARGE/DISCHARGE CURVE, invented by Yuhao Lu et al., Ser. No.13/752,930, filed Jan. 29, 2013, attorney docket No. SLA3265;

which is a Continuation-in-Part of an application entitled,SUPERCAPACITOR WITH HEXACYANOMETALLATE CATHODE, ACTIVATED CARBON ANODE,AND AQUEOUS ELECTROLYTE, invented by Yuhao Lu et al., Ser. No.13/603,322, filed Sep. 4, 2012, attorney docket No. SLA3212,

Ser. No. 13/752,930 is also a Continuation-in-Part of an applicationentitled, IMPROVEMENT OF ELECTRON TRANSPORT IN HEXACYANOMETALLATEELECTRODE FOR ELECTROCHEMICAL APPLICATIONS, invented by Yuhao Lu et al.,Ser. No. 13/523,694, filed Jun. 14, 2012, attorney docket No. SLA3152;

which is a Continuation-in-Part of an application entitled, ALKALI ANDALKALINE-EARTH ION BATTERIES WITH HEXACYANOMETALLATE CATHODE ANDNON-METAL ANODE, invented by Yuhao Lu et al, Ser. No. 13/449,195, filedApr. 17, 2012, attorney docket no. SLA3151;

which is a Continuation-in-Part of an application entitled, ELECTRODEFORMING PROCESS FOR METAL-ION BATTERY WITH HEXACYANOMETALLATE ELECTRODE,invented by Yuhao Lu et al., Ser. No. 13/432,993, filed Mar. 28, 2012,attorney docket no. SLA3148. All these applications are incorporatedherein by reference.

This invention was made with Government support under DE-AR0000297awarded by DOE. The Government has certain rights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to electrochemical cells and, moreparticularly, to a sodium iron(II)-hexacyanoferrate(II) material,iron(II)-hexacyanoferrate(II) battery electrode, and associatedfabrication processes.

2. Description of the Related Art

A battery is an electrochemical cell through which chemical energy andelectric energy can be converted back and forth. Overall, the energydensity of a battery is determined by its voltage and charge capacity.Lithium has the most negative potential (−3.04 V vs. H₂/H⁺), andexhibits the highest gravimetric capacity corresponding to 3860milliamp-hours per gram (mAh/g). Due to their high energy densities,lithium-ion batteries (LIBs) have triggered the portable electronicsrevolution. However, both the high cost of lithium metal and the strainon natural resources render doubtful the commercialization of LIBs aslarge scale energy storage devices. In general, LIBs employ lithiumstorage compounds as the positive (cathode) and negative (anode)electrode materials. During battery cycling, lithium ions (Li⁺) areexchanged between the positive and negative electrodes. LIBs have beenreferred to as “rocking chair” batteries since the lithium ions “rock”(shuttle) back and forth between the positive and negative electrodes asthe cells are charged and discharged. The positive electrode (cathode)material is conventionally a metal oxide with a layered structure, suchas lithium cobalt oxide (LiCoO₂), or a material having a tunneledstructure, such as lithium manganese oxide (LiMn₂O₄), on an aluminumcurrent collector. The negative electrode (anode) typically consists ofgraphitic carbon, also a layered material, on a copper currentcollector. During the charge-discharge process, lithium ions areinserted into, or extracted from, the interstitial spaces of the activematerials.

Analogous to LIBs, metal-ion batteries employ metal-ion host compoundsas their electrode materials into which metal-ions can migrate botheasily and reversibly. Since Li⁺ has one of the smallest radii amongmetal ions, it is easily accommodated within the interstitial spaces ofvarious materials including layered LiCoO₂, olivine-structured LiFePO₄,spinel-structured LiMn₂O₄, and so on. In contrast, larger metal ionssuch as sodium ions (Na⁺), potassium ions (K⁺), magnesium ions (Mg²⁺),aluminum ions (Al³⁺), zinc ions (Zn²⁺), etc., severely distort thestructures of conventional Li+ intercalation materials and,consequently, destroy the host structures within severalcharge/discharge cycles. In light of this, new materials with largerinterstitial spaces are required in order to accommodate variousmetal-ions for a metal-ion battery.

FIG. 1 is a diagram depicting the crystal structure of a transitionmetal hexacyanoferrate (TMHCF) in the form of A_(x)M1M2(CN)₆ (priorart). Transition metals are defined as elements whose atoms possess anincomplete d sub-shell or can give rise to cations (transition metalions) with an incomplete d shell and include Groups 3 to 12 of thePeriodic Table. The crystal structure of TMHCFs exhibits an openframework and is analogous to that of the ABXs perovskite, as shown. Ingeneral, M₁ and M₂ are transition metal ions in an ordered arrangementon the B sites. The large, tetrahedrally coordinated A sites can hostboth alkali and alkaline earth ions (A_(x)) in addition to species suchas H₂O. The number of alkali (or alkaline earth ions) in the large cagesof this crystallographically porous framework may vary from x=0 to x=2depending on the respective valence(s) of M₁ and M₂. Conveniently, theopen framework structure of the TMHCFs facilitates both rapid andreversible intercalation processes for alkali and alkaline earth ions(A_(x)).

Transition metal hexacyanoferrates (TMHCFs) with large interstitialspaces have been investigated as cathode materials for rechargeablelithium-ion batteries,^([1, 2]) sodium-ion batteries,^([3, 4]) andpotassium-ion batteries,^([5]) By employing an aqueous electrolytecontaining the appropriate alkali-ions or ammonium-ions, copper andnickel hexacyanoferrates [(Cu,Ni)-HCFs] demonstrated a robust cyclinglife with 83% capacity retention after 40,000 cycles at acharge/discharge current rate of 17C.^([6-8]) In spite of this, thematerials demonstrated low capacities and energy densities due to thefacts that (1) only one sodium-ion (Na⁺) could be inserted/extractedinto/from per Cu-HCF or NrHCF formula, and (2) the TM-HCFs electrodeswere restricted to operation below 1.23 volts (V) due to theelectrochemical window for water decomposition. In order to compensatefor such shortcomings, manganese hexacyanoferrate (Mn-HCF) and ironhexacyanoferrate (Fe-HCF) were employed as cathode materials innon-aqueous electrolyte systems.^([9, 10]) When assembled into batterieswith sodium-metal anode, Mn-HCF and Fe-HCF electrodes deliveredcapacities of ˜110 mAh/g when cycled between 2.0 and 4.2 V.

It is worth noting that it is extremely difficult to directly obtainNa₂Fe₂(CN)₆ through a conventional precipitation method. Typically, uponaddition of an Fe²⁺-containing solution into a solution of Fe(CN)₆ ⁴⁻ ,Fe²⁺-ions are immediately oxidized to afford a blue precipitate ofNa_(1-x)Fe₂(CN)₆. Electrochemical methods have been used to determinethat X equals 0.48 for a Na_(1-x)Fe₂(CN)₆ sample synthesized in thismanner. Furthermore, the small Na⁺ content confirms that a certainproportion of Fe(II) in Fe(CN)₆ ⁴⁻ was similarly oxidized during theprocess. In 2011, Hu et al. reported a hydrothermal method to synthesizeK₂Fe₂(CN)₆ from K₄Fe(CN)₆ in a neutral pH solution.^([11]) However, itis difficult to synthesize Na₂Fe₂(CN)₆ due to the fact that sodium ionsare smaller than potassium ions and are therefore harder to retain inthe large interstitial space of Fe-HCF. In addition, the reaction issensitive to the pH of the reaction solution.^([12-14]) In acidicsolutions (pH<7), K₄Fe(CN)₆ produced a Prussian blue material, namelyKFe₂(CN)₆, using a hydrothermal reaction process. In alkaline solutions(pH>7), K₄Fe(CN)₆ decomposed and formed iron (II, III) oxide (Fe₃O₄). Inlight of these results, it may be surmised that the formation ofNa₂Fe₂(CN)₆ requires a specific solution pH unlike the neutralconditions that were found to be satisfactory for synthesizingK₂Fe₂(CN)₆. However, the deliberate adjustment of the pH for reactionsolutions also failed to produce Na₂Fe₂(CN)₆.

It would be advantageous if a process existed that was able to directlysynthesize Na_(1+X)Fe[Fe(CN)₆], where X is less than or equal to 1.

[1]V. D. Neff, “Some Performance Characteristics of a Prussian BlueBattery”, Journal of Electrochemical Society 1985. 132, 1382-1384.

[2]N. Imanishi, T. Morikawa, J. Kondo, Y. Takeda, O.Yamamoto, N.Kinugasa, and T. Yamagishi, “Lithium Intercalation Behavior into IronCyanide Complex as Positive Electrode of Lithium Secondary Battery”,Journal of Power Sources 1999, 79, 215-219.

[3]Y. Lu, L. Wang, J. Cheng, and J. B. Goodenough, “Prussian Blue: a NewFramework for Sodium Batteries”, Chemistry Communications 2012, 48,6544-6546.

[4]L. Wang, Y. Lu, J. Liu, M. Xu, J. Cheng, D. Zhang, and J. B.Goodenough, “A Superior Low-Cost Cathode for a Na-ion Battery”,Angewandte Chemie International Edition 2013, 52, 1964-1967.

[5]A. Eftekhari, “Potassium Secondary Ceil Based on Prussian BlueCathode”, Journal of Power Sources 2004, 126, 221-228.

[6]C. D. Wessells, R. A. Huggins, and Y. Cui, “Copper HexacyanoferrateBattery Electrodes with Long Cycle Life and High Power”, NatureCommunications 2011, 2, Article number: 550.

[7]C, D. Wessells, S. V. Peddada, R. A. Huggins, and Y. Cui, “NickelHexacyanoferrate Nanoparticle Electrodes for Aqueous Sodium andPotassium Ion Batteries”, Nano Letters 2011, 11, 5421-5425.

[8]C. D. Wessells, S. V. Peddada, M. T. McDowell, R. A. Huggins, and Y.Cui, “The Effect of Insertion Species on Nanostructured Open FrameworkHexacyanoferrate Battery Electrodes”, Journal of the ElectrochemicalSociety 2012, 159, A98-A103.

[9]T. Matsuda, M. Takaehi, and Y. Moritomo, “A Sodium ManganeseFerrocyanide Thin Film for Nation Batteries”, Chemical Communications2013, 49, 2750-2752.

[10]S-H. Yu, M. Shokouhimehr, T. Hyeon, and Y-E. Sung, “IronHexacyanoferrate Nanoparticles as Cathode Materials for Lithium andSodium Rechargeable Batteries”, ECS Electrochemistry Letters 2013, 2,A39-A41.

[11]M. Hu and J. S. Jiang, “Facile Synthesis of Air-Stable PrussianWhite Microcubes via a Hydrothermal Method”, Materials Research Bulletin2011, 48, 702-707.

[12]S-H. Lee and Y-D. Huh, “Preferential Evolution of Prussian Blue'sMorphology from Cube to Hexapod”, The Bulletin of the Korean ChemicalSociety 2012, 33, 1078-1080.

[13]M. Hu, J-S. Jiang, C-C. Lin, and Y. Zeng, “Prussian BlueMesocrystals: an Example of Self-Construction”, CrystEngComm 2010, 12,2879-2683.

[14]M. Hu, R-P. Ji, and J-S. Jiang, “Hydrothermal Synthesis of MagnetiteCrystals: from Sheet to Pseudo-Octahedron”, Materials Research Bulletin2010, 45, 1811-1715.

SUMMARY OF THE INVENTION

Described herein is a hydrothermal reaction process for directlysynthesizing sodium iron(II)-hexacyanoferrate(II) (Na_(1+X)Fe[Fe₂(CN)₆]). According to one aspect of this method, reducingagents and/or antioxidants are integrated into the reaction solutions.During the hydrothermal reaction, Fe(II) in the reaction solution isprotected from oxidization by the presence of reducingagents/anti-oxidants. Unlike NaFe₂(CN)₆, Na_(1+X)Fe[Fe₂(CN)₆]can bedirectly used as a cathode in sodium-ion batteries with a non-sodiumanode to achieve high capacity. Some unique aspects of the processinclude:

-   -   (1) Hydrothermal reaction processes and conditions are employed        to synthesize sodium iron(II) hexacyanoferrate        [Na_(1+X)Fe[Fe(CN)₆]], where X=0 to 1.    -   (2) Reducing agents or antioxidants are included within the        hydrothermal reaction solutions to protect iron(II) from        oxidization, which allows more sodium-ions to be retained in the        Fe-HCF structure.    -   (3) The reaction solution can be weakly acidic, although it is        not necessary.    -   (4) Without any additional processes, Na_(1+X)Fe[Fe(CN)₆]can be        directly employed as a cathode material in sodium-ion batteries        with a non-sodium metal anode.

Accordingly, a method is provided for synthesizing sodiumiron(II)-hexacyanoferrate(II). The method prepares a first solutionincluding sodium ions, where the sodium ions are derived from a materialsuch as sodium nitrite, sodium nitrate, sodium chloride, sodiumcarbonate, sodium acetate, sodium phosphate, sodium thiosulfate, sodiumiodide, sodium bisulfite, sodium sulfite, sodium bromide, sodiumfluoride, or combinations thereof. The first solution may include anaqueous solvent, non-aqueous solvent, or a combination thereof.

A Fe(CN)₆ material is mixed with the first solution and either anantioxidant or a reducing agent. The Fe(CN)₆ material may be eitherferrocyanide ([Fe(CN)₆]⁴⁻) or ferricyanide ([Fe(CN)₆]³⁻). Potentialantioxidants and reducing agents include monosaccharides, disaccharides,glucose, ascorbic acid, formic acid, alcohols, oxalic acid, aldehydes,ketones, organic compounds having reducing properties, inorganiccompounds having reducing properties, or combinations thereof.

As a result, sodium iron(II)-hexacyanoferrate(II)(Na_(1+X)Fe[Fe(CN)₆]_(Z).MH₂O) is formed, where X are Z are each lessthan or equal to 1, and where M is in a range between 0 and 7. In oneaspect, the first solution including includes A ions, such as alkalimetal ions, alkaline earth metal ions, or combinations thereof,resulting in the formation of Na_(1+X)A_(Y)Fe[Fe(CN)₆]_(Z).MH₂O, where Yis less than or equal to 1.

Additional details of the above-described method, aNa_(1+X)Fe[Fe(CN)₆]_(Z) battery electrode, and Na_(1+X)Fe[Fe(CN)₆]_(Z)battery are presented below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram depicting the crystal structure of a transitionmetal hexacyanoferrate (TMHCF) in the form of A_(x)M1M2(CN)₆ (priorart).

FIG. 2 is a partial cross-sectional view of a sodiumiron(II)-hexacyanoferrate(II) battery electrode.

FIGS. 3A and 3B are, respectively, schematic drawings ofNa_(1+X)Fe[Fe(CN)₆]_(Z).MH₂O and Na_(1+X)A_(Y)Fe[Fe(CN)₆]Z.MH₂O.

FIG. 4 is a partial cross-sectional view of a battery employing thebattery electrode of FIG. 2.

FIG. 5 is a graph representing the electrochemical behavior ofNa_(1+X)Fe[Fe(CN)₆]_(Z) synthesized by a conventional precipitationmethod that mixes a Fe²⁺ solution and a ferrocyanide solution.

FIG. 8 is an x-ray diffraction (XRD) pattern of a NaFe₂(CN)₆sampleprepared using a method purportedly able to produce K₂Fe₂(CN)₆.

FIG. 7 is a schematic diagram depicting the structure of ascorbic acid,which is a common antioxidant.

FIG. 8 is a XRD pattern for Na_(1+X)Fe[Fe(CN)₆]z synthesized using ahydrothermal process including ascorbic acid in the reaction solution.

FIG. 9 is a graph depicting the charge/discharge curves for batteriesusing Na_(1+X)Fe[Fe(CN)₆]z material synthesized using a hydrothermalprocess including ascorbic acid in the reaction solution.

FIG. 10 is a flowchart illustrating method for synthesizing sodiumiron(II)-hexacyanoferrate(II).

DETAILED DESCRIPTION

FIG. 2 is a partial cross-sectional view of a sodiumiron(II)-hexacyanoferrate(II) battery electrode. The battery electrode200 comprises a current collector 202 and sodiumiron(II)-hexacyanoferrate(II) (Na_(1+X)Fe[Fe(CN)₆]_(Z).MH₂O) 204overlying the current collector,

-   -   where X and Z are each less than or equal to 1; and,    -   where M is in a range of 0 to 7.

In one aspect, the sodium iron(II)-hexacyanoferrate(II) 204 additionallycomprises A ions such as alkali metal ions, alkaline earth metal ions,or combinations thereof, forming Na_(1+X)A_(Y)Fe[Fe(CN)₆]Z.MH₂O, where Yis less than or equal to 1. The A ions may be lithium ions (Li⁺), sodiumions (Na⁺), potassium ions (K⁺), rubidium ions (Rb⁺), cesium ions (Cs⁺),beryllium ions (Be⁺), magnesium ions (Mg⁺), calcium ions (Ca⁺),strontium ions (Sr⁺), or barium ions (Ba⁺). As used herein, alkali metalrefers to elements in Group 1 of the Periodic Table which exhibit atendency to form ions with a single positive charge (alkali metal ions)through loss of an electron. Alkaline earth metals include thoseelements in Group 2 of the Periodic Table which readily lose twoelectrons to form species with a 2+ charge (alkaline earth metal ions).

As would be understood by those with ordinary skill in the art, thebattery electrode 200 may also include a conductor (not shown), such asa carbonaceous material including carbon black, carbon nanotubes, carbonfibers, etc., to improve electrical conductivity between the sodiumiron(II)-hexacyanoferrate(II) 204, and between the sodiumiron(II)-hexacyanoferrate(II) 204 and the current collector 202. Thebattery electrode 200 may also include a polymeric binder (not shown),such as polytetrafluoroethylene (PTFE) or polyvinylidene difluoride(PVDF) to provide adhesion between electrode components/currentcollector and improve the overall physical stability and form of thebattery electrode.

FIGS. 3A and 3B are, respectively, schematic drawings ofNa_(1+X)Fe[Fe(CN)₆]_(Z).MH₂O and Na_(1+X)A_(Y)Fe[Fe(CN)₆]Z.MH₂O.

FIG. 4 is a partial cross-sectional view of a battery employing thebattery electrode of FIG. 2. The battery 400 comprises a cathode(positive) battery electrode 401, an electrolyte 402, and an anode(negative) battery electrode 406. Either the cathode battery electrode401 or the anode battery electrode 406 is made from sodiumiron(II)-hexacyanoferrate(II) overlying a current collector, asdescribed above in the explanation of FIG. 2. For example, the cathode401 can be made from a sodium iron(II)hexacyanoferrate(II) materialwhile the anode 406 is made with a TMHCF material other than sodiumiron(II)-hexacyanoferrate(II). The designation of an electrode as ananode or a cathode is arbitrary, based upon the relative potentials ofthe two TMHCF materials. The electrolyte 402 may be non-aqueous, such asan organic liquid electrolyte, or alternatively, gel electrolyte,polymer electrolyte, solid (inorganic) electrolyte, etc. Common examplesof non-aqueous (liquid) electrolytes include organic carbonates such asethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate(DEC), etc., although many other organic carbonates and alternatives toorganic carbonates exist. Typically, gel electrolytes consist ofpolymeric materials which have been swelled in the presence of liquidelectrolytes. Examples of polymers employed as gel electrolytes include,but are not limited to, poly(ethylene)oxide (PEO) and fluorinatedpolymers such as poly(vinylidene) fluoride (PVDF)-based polymers andcopolymers, etc. In contrast, (solid) polymer electrolytes may beprepared using the same classes of polymers for forming gel electrolytesalthough swelling of the polymer in liquid electrolytes is excluded.Finally, solid inorganic (or ceramic) materials may be considered aselectrolytes, which may be employed in combination with liquidelectrolytes. Overall, the appropriate electrolyte system may consist ofcombinations (hybrid) of the above classes of materials in a variety ofconfigurations. Otherwise, an aqueous electrolyte compatible with thesodium iron(II)-hexacyanoferrate(II) may be used. An ion-permeablemembrane 404 is interposed between the cathode 200 a and the anode 200b. In some instances not shown, the ion-permeable membrane and theelectrolyte can be the same material, as may be the case for polymergel, polymer, and solid electrolytes.

In another aspect, if the cathode battery electrode 401 is made fromsodium iron(II)-hexacyanoferrate(II), as described above, the anodebattery electrode 406 may be made from a sodium metal, a metal (otherthan sodium), metal alloy, non-metal material, and/or a polymermaterial.

The electrodes described above are the result of an improvedhydrothermal reaction strategy to synthesize sodiumiron(II)-hexacyanoferrate (Na_(1+X)Fe[Fe(CN)₆]_(Z)). In general, metalhexacyanoferrates can be synthesized in a straightforward method using astandard precipitation method. Briefly, a solution containing metal-ionsis dropped (added) into a ferrocyanide/ferricyanide solution to affordthe metal hexacyanometallate material. Conventionally, the precipitationmethod is not readily applicable to the preparation of Fe(II)hexacyanoferrate (II) (Fe-HCF) synthesis since Fe(II) is readilyoxidized in the solution even under inert (nitrogen) atmosphere.

FIG. 5 is a graph representing the electrochemical behavior ofNa_(1+X)Fe[Fe₂(CN)₆]_(Z) synthesized by a conventional precipitationmethod that mixes a Fe²⁺ solution and a ferrocyanide solution. Duringthe first charge, the material demonstrated a capacity of 40.85 mAh/g,which corresponds to 0.52 Na⁺-ions per formula. Furthermore,thermogravimetric analysis (TGA) analysis confirmed the presence of 2.79water molecules per formula. Thus, the molecular formula can becalculated to be Na_(0.52)Fe^(III)[Fe^(II)(CN)₆]_(1-y).2.79H₂O (y<1). Asa result, it is evident that Na_(1+X)Fe[Fe₂(CN)6] cannot be synthesizedusing the conventional precipitation method.

FIG. 6 is an x-ray diffraction (XRD) pattern of a NaFe₂(CN)₆sampleprepared using a method purportedly able to produce K₂Fe₂(CN)6. At thetime of this filing, only one technical paper has been found whichdescribed the synthesis of a stable potassium Fe(II)-HCF(II)(K₂Fe₂(CN)₆) using the hydrothermal reaction.^([11]) Experiments wereperformed with strict adherence to the reported methods in attempts tosynthesize sodium Fe(II)-HCF(II) (Na₂Fe₂(CN)₆). However, nowhite-colored product consistent with Na₂Fe₂(CN)₆ was obtained from theprocess. In fact, the product was determined to be a mixture ofNaFe₂(CN)₆, Fe₃O₄, and iron(III) oxide (Fe₂O₃), although all peaks ofFe₂O₃ exhibited a small displacement to the low angles. Somedisplacement is normal in XRD experiments, due to instrument accuracyand calibration, sample surface irregularities, and changes in the sizeof the crystal under examination. As contrast to the XRD pattern of thesynthesized NaFe₂(CN)₆ sample (top), showing peaks and peak locations,the standard (known) peak locations for Fe₃O₄ (middle) and Fe₂O₃(bottom) are shown.

Additional experiments were also performed during which the pH of thereaction solution was deliberately adjusted to slightly acidic (pH<7),while oxygen was removed from the reaction solution by purging withnitrogen, with the expectation that Prussian white [Na₂Fe₂(CN)₆] couldbe synthesized under these conditions. Despite pH values of less than 5,it was observed that as the pH decreased, the corresponding colors ofthe reaction products transitioned from red (Fe₂O₃) to dark blue(Prussian blue). However, no white-colored product consistent with theformation of Na₂Fe₂(CN)₆ was observed.

The above results clearly demonstrate the fact that Fe(II)-ions wereoxidized during the reaction regardless of solution pH. In response tothis, an improved hydrothermal reaction was developed, as disclosedherein, to synthesize sodium Fe(II)-HCF(II) by introducing reducingagents or antioxidants to the reaction medium. During the hydrothermalreaction, these agents provide a reducing environment to protect Fe(II)from oxidization.

FIG. 7 is a schematic diagram depicting the structure of ascorbic acid,which is a common anti-oxidant. In one experiment, ascorbic acid wasadded into the hydrothermal reaction solution. From a mechanisticperspective, ascorbic acid ions react with oxygen radicals (O^()) toeffectively remove oxidizing species from the reaction solutionaccording to the following:

O^(*)+C₆H₇O₆ ⁻→OH⁻+C₆H₆O₆ ^(*−)

FIG. 8 is a XRD pattern for sodium Fe(II)-HCF(II) synthesized using ahydrothermal process including ascorbic acid in the reaction solution.Accordingly, Fe(II) was protected from oxidation and sodiumFe(II)-HCF(II) was formed. Unlike NaFe₂(CN)₆, with a cubic structure,sodium Fe(II)-CHF(II) exhibits a rhombohedral structure. The presence ofadditional sodium-ions in the framework introduces a rhombohedral sitesymmetry along each of the four [1 1 1] axes that stabilize thesodium-ion displacement along a cubic [1 1 1] axis toward a morenegative octahedral-site complex. This rhombohedral structure is alsodifferent from potassium Fe(II)-HCF(II) reported by Hu, et al. whereinthe K₂Fe₂(CN)₆ exhibited a monoclinic structure.^([11])

FIG. 9 is a graph depicting the charge/discharge curves for batteriesusing Na_(1+X)Fe[Fe(CN)₆]_(Z) material synthesized using a hydrothermalprocess including ascorbic acid in the reaction solution. Based upon afirst charge capacity of 135 mAh/g, the number of sodium could becalculated to be ˜1.6 per formula. In other words, the formula of sodiumFe(II)-HCF(II) could be designated as Na_(1.6)Fe[Fe(CN)₆].MH₂O. Theseresults unambiguously demonstrate that sodium Fe(II)-HCF(H) can bereadily synthesized via the modified hydrothermal reaction whereinreducing agents and antioxidants assume a critical role in protectingFe(II) from oxidization during the process. Unlike NaFe₂(CN)₆,Na_(1+X)Fe[Fe(CN)₆]_(Z).MH₂O with a high Na⁺ content can be directlyemployed as cathode materials in sodium-ion batteries using a non-sodiummetal anode.

FIG. 10 is a flowchart illustrating method for synthesizing sodiumiron(II)-hexacyanoferrate(II). Although the method is depicted as asequence of numbered steps for clarity, the numbering does notnecessarily dictate the order of the steps. It should be understood thatsome of these steps may be skipped, performed in parallel, or performedwithout the requirement of maintaining a strict order of sequence.Generally however, the method follows the numeric order of the depictedsteps. The method starts at Step 1000.

Step 1002 prepares a first solution including sodium ions. The sodiumions may be derived from materials such as sodium nitrite, sodiumnitrate, sodium chloride, sodium carbonate, sodium acetate, sodiumphosphate, sodium thiosulfate, sodium iodide, sodium bisulfite, sodiumsulfite, sodium bromide, sodium fluoride, or combinations thereof. Step1004 mixes a Fe(CN)₆ material with the first solution and a firstelement, which is either an anti-oxidant or a reducing agent. TheFe(CN)₆ material may be either ferrocyanide ([Fe(CN)₆]⁴⁻) orferricyanide ([Fe(CN)₆]³⁻). As used herein, an anti-oxidant is definedas a material that inhibits oxidation. Antioxidants function to removefree radical intermediates generated during oxidation processes as wellas inhibit other oxidation reactions. A reducing agent is defined as aspecies that donates an electron to another species in anoxidation-reduction (redox) reaction. Consequently, the reducing agentis itself oxidized in the process since electron(s) have been forfeited.Overall, there exist a number of agents that can function asanti-oxidants and/or reducing agents including monosaccharides (glucose,glyceraldehyde, galactose), disaccharides (lactose, maltose), starch,ascorbic acid, formic acid, alcohols, oxalic acid, aldehydes, ketones,organic compounds having reducing properties, and inorganic compoundshaving reducing properties. In general, reducing sugars (reducing mono-and di-saccharides) are characterized by “open-chain” forms containingan aldehyde group or, alternatively, containing a ketone group that canprovide an aldehyde group via isomerization. The aldehyde group of thereducing sugar can be oxidized via a redox reaction through which aseparate material is reduced in the process. Starches are glucosepolymers that may contain an abundance of aldehyde groups. Ascorbic acidis a naturally occurring anti-oxidant that represents a form of VitaminC. Since formic acid in a deprotonated form breaks into hydride andcarbon dioxide, it can function as a reducing agent. Considering thefact that alcohols are sensitive to oxidation to yield aldehydes,ketones and/or carboxylic acids depending upon the nature of the alcoholand/or reaction conditions, they can be viewed as behaving as reducingagents. Oxalic acid can function as a reducing agent by donatingelectrons during which process it is transformed (oxidized) to carbondioxide.

Step 1006 forms sodium iron(II)-hexacyanoferrate(II)(Na_(1+X)Fe[Fe(CN)₆]_(Z).MH₂O),

-   -   where X and Z are each less than or equal to 1; and,    -   where M is in a range between 0 and 7.

In one aspect, preparing the first solution in Step 1002 includes thefirst solution comprising a solvent such as an aqueous solvent,non-aqueous solvent, or combinations thereof. In another aspect, mixingthe Fe(CN)₆ material with the first solution and the first element inStep 1004 includes heating at a temperature in a range between about 20and 1000° C., forming an intermediate product. The mixing of the Fe(CN)₆material with the first solution and the first element may occur for aduration in the range of 1 hour to 1 month. Further, the mixing mayperformed by stirring, agitating, or shaking.

In one variation, subsequent to forming the intermediate product, Step1005 a washes the intermediate product in a solution such as an aqueoussolution, non-aqueous solution, or combinations thereof. Step 1005 bdries the intermediate product at a temperature in a range between about20 and 200° C. In one aspect, Step 1005 b dries the intermediate productunder vacuum at a pressure in a range between 0.001 mTorr and 30 Torr.In one aspect, the separation of the intermediate product from the washsolution prior to drying may be performed using a number of conventionalprocesses including, but not limited to, filtration and centrifugation.

In another aspect, preparing the first solution in Step 1002 includesthe first solution comprising A ions such as alkali metal ions, alkalineearth metal ions, or combinations thereof. As a result, Step 1006 formsNa_(1+X)A_(Y)Fe[Fe(CN)₆]_(Z).MH₂O, where Y is less than or equal to 1.The A ions may be lithium ions (Li⁺), sodium ions (Na⁺), potassium ions(K⁺), rubidium ions (Rb⁺), cesium ions (Cs⁺), beryllium ions (Be⁺),magnesium ions (Mg⁺), calcium ions (Ca⁺), strontium ions (Sr⁺), orbarium ions (Ba⁺).

A Na_(1+X)Fe[Fe(CN)₆] battery electrode and associated synthesisprocesses have been provided. Examples of particular materials andprocess steps have been presented to illustrate the invention. However,the invention is not limited to merely these examples. Other variationsand embodiments of the invention will occur to those skilled in the art.

We claim: 1-12. (canceled)
 13. A sodium iron(II)-hexacyanoferrate(II)battery electrode, the battery electrode comprising: a currentcollector; sodium iron(II)-hexacyanoferrate(II)(Na_(1+X)Fe[Fe(CN)₆]_(Z).MH₂O) overlying the current collector, where Xand Z are each less than or equal to 1; and, where M is in a range of 0to
 7. 14. The battery electrode of claim 13 wherein the sodiumiron(II)-hexacyanoferrate(II) additionally comprises A ions selectedfrom the group consisting of alkali metal ions, alkaline earth metalions, and combinations thereof, formingNa_(1+X)A_(Y)Fe[Fe(CN)₆]_(Z).MH₂O, where Y is less than or equal to 1.15. The battery electrode of claim 14 wherein the A ions are selectedfrom the group consisting of lithium ions (Li⁺), sodium ions (Na⁺),potassium ions (K⁺), rubidium ions (Rb⁺), cesium ions (Cs⁺), berylliumions (Be⁺), magnesium ions (Mg⁺), calcium ions (Ca⁺), strontium ions(Sr⁺), and barium ions (Ra⁺).
 18. A sodium iron(II)-hexacyanoferrate(II)battery comprising: a cathode comprising: a current collector; sodiumiron(II)-hexacyanoferrate(II) (Na_(1+X)Fe[Fe(CN)₆]_(Z).MH₂O, overlyingthe current collector, where X AND Z are each less than or equal to 1;where M is in a range of 0 to 7; an anode; an electrolyte; and, anion-premeable membrane.
 17. The battery of claim 16 wherein the sodiumiron(II)-hexacyanoferrate(II) additionally comprises A ions selectedfrom the group consisting of alkali metal ions, alkaline earth metalions, and combinations thereof, formingNa_(1+X)A_(Y)Fe[Fe(CN)₆]_(Z).MH₂O, where Y is less than or equal to 1.18. The battery of claim 17 wherein the A ions are selected from thegroup consisting of lithium ions (Li⁺), sodium ions (Na⁺), potassiumions (K⁺), rubidium ions (Rb⁺), cesium ions (Cs⁺), beryllium ions (Be⁺),magnesium ions (Mg⁺), calcium ions (Ca⁺), strontium ions (Sr⁺), andbarium ions (Ba⁺).